This is introduced in C99 standard called designated initializer.
struct
{
int a[2], b;
} arr[] = {[0].a = {1}, [1].a = {2}, [0].b = 1, [1].b = 2};
This portion we can write as
struct
{
....
}
arr[0].a[0]=1
arr[0].a[1]=0
arr[0].b=1
arr[1].a[0]=2
arr[1].a[1]=0
arr[1].b=2
In structure declaration , here are three elements. Array contains 2 elements and another integer b contains 1 element.
So, no compiler error will be there. Answer will be C)